1. Field of the Invention
The present invention generally relates to broadcast communication such as amplitude, frequency or pulse modulation radio and television and, more particularly, to broadcast carrier frequency usage and synchronization, particularly for translators used for local broadcast coverage.
2. Description of the Prior Art
Broadcast communications have become an important and rapidly growing industry. During the past few years, many new frequencies and channels have been allocated to provide an increased variety of programming. Many new broadcast stations have also been recently established to improve broadcast coverage to more remote receiver locations. However, coverage has not yet become complete in the United States, in large part due to mountain ranges and the fact that FM radio and television frequencies are not reflected from upper layers of the atmosphere and thus require substantially a line-of-sight transmission path for reliable communications to be carried out. Even in urban areas, some locations will be able to achieve only marginal reception due to reflections from buildings, aircraft and the like causing so-called multi-path distortion and signal cancellation.
Of course, different programming cannot be broadcast on the same channel or frequency if the transmission patterns overlap since a receiver in a region of such overlap could not easily reject either signal. For this reason, at the present time, the transmission pattern of broadcast transmitters may require modification with specially designed antenna arrays. Further, broadcast transmitters using the same channel or frequency are not permitted within one hundred seventy miles of each other and transmission power is closely regulated. Further, the carrier frequencies of respective transmitters in a triad (a group of three transmitters using a frequency or channel which is nominally the same and which are most proximate to each other) are offset from each other by 10 KHz or 20 KHz under present frequency allocation regulations of the Federal Communications Commission to facilitate discrimination. However, when coverage overlaps, beating of carriers may seriously degrade reception of either signal.
It is customary at the present time to deal with areas of poor coverage within the allocated broadcast pattern of broadcast stations by the use of so-called translators functioning as a relay station. A translator is essentially a strategically located slave transmitter which receives a broadcast signal on one or more frequencies or channels and provides retransmission on other frequencies or channels. The term translator derives from the function of translating the modulation of a received signal to a different carrier signal. For the reasons discussed in the preceding paragraph, the frequencies or channels on which broadcast signals are received cannot be used for retransmission since interference with one or more of the original broadcast signals would otherwise result. The translators which may be used are also subject to regulations as to minimum geographical separation, transmission pattern and frequency offset as broadcast stations. As an additional complicating factor, frequency allocation regulations allow deviation of carrier frequency of broadcast transmitters 50 Hz above or below a nominal carrier frequency.
The overall effect of the above-described circumstances is to favor usage of a substantial fraction of the available frequencies and channels for enhancement of coverage pattern of broadcast stations rather than to provide the variety of programming which might otherwise be possible. Further, even though many marginal reception areas are serviced by translators, the frequency offsets described above reduce the signal strength. That is, discrimination would be increased by 11 db if frequency offsets were not employed.
It should also be recognized that the same problems, while substantially less severe, are also present for amplitude modulated signals at much lower frequencies for which a line-of-sight transmission path is not as critical. Nevertheless, amplitude or pulse modulated signals are much more subject to interference and noise as well as exhibiting areas of poor broadcast signal coverage.
Accordingly, the frequency allocation arrangement currently in use, while based on sound technical requirements, tends to diminish signal quality and limit the number of channels which can be used in any given geographical area. This, in turn, limits the amount of information which can be concurrently communicated at a time when frequency allocations are at a premium and demand for access to information and programming variety are rapidly increasing.
It is therefore an object of the present invention to provide an inexpensive system for synchronizing broadcast facilities to avoid the need for frequency offsets and the need for use of different frequencies by relay station translators.
It is another object of the invention to provide for enhancement of broadcast signals consistent with the avoidance of interference between broadcast signals.
It is a further object of the invention to provide for full utilization of channel allocations in support of diversity of programming.
It is yet another object of the invention to provide, at low cost, a system permitting effective directional discrimination, if permitted, between stations broadcasting on a common frequency.
In order to accomplish these and other objects of the invention, a method of synchronizing a plurality of broadcast transmitters is provided, including steps of relaying a synchronization tone to a plurality of transmitters by microwave tone, multiplying the frequency of the synchronization tone to obtain a desired carrier frequency at each of the plurality of transmitters, and modulating the carrier frequency simultaneously at the plurality of broadcast transmitters with a modulation signal.
In accordance with another aspect of the invention, a broadcast transmitter system is provided including a plurality of transmitters having a common allocated channel or nominal carrier frequency, each respective transmitter including a receiver for receiving a tone transmitted from a broadcast transmitter, a frequency multiplier, preferably formed of one or more full-wave rectifier stages and tuned filters, for multiplying the frequency of the tone to derive a carrier frequency, and a modulator for modulating the carrier frequency with a broadcast signal.